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// Richard CPU
// Copyright (C) 2026 Richard Qin
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.
// File path: rtl/core/hazard_unit.sv
// Description: Hazard Control Unit
// Handles data hazards (forwarding, Load-Use stalls), control hazards
// (branch misprediction flushes), and memory stalls (D-Cache misses).
module hazard_unit
import rv64_pkg::*;
(
// -----------------------
// Inputs from ID Stage (for Load-Use detection)
// -----------------------
input logic [4:0] id_rs1_addr,
input logic [4:0] id_rs2_addr,
// -----------------------
// Inputs from EX Stage (ID/EX register outputs)
// -----------------------
input logic [4:0] ex_rs1_addr,
input logic [4:0] ex_rs2_addr,
input logic [4:0] ex_rd_addr,
input logic ex_reg_write_en,
input logic ex_is_load,
input logic ex_branch_taken,
// -----------------------
// Inputs from MEM Stage (EX/MEM register outputs)
// -----------------------
input logic [4:0] mem_rd_addr,
input logic mem_reg_write_en,
input logic mem_stall_req, // D-Cache miss or memory busy
// -----------------------
// Inputs from WB Stage (MEM/WB register outputs)
// -----------------------
input logic [4:0] wb_rd_addr,
input logic wb_reg_write_en,
// -----------------------
// Outputs: EX-stage operand forwarding mux selects
// -----------------------
// 00: no forward (use regfile), 01: forward from MEM, 10: forward from WB
output logic [1:0] forward_a_sel,
output logic [1:0] forward_b_sel,
// -----------------------
// Outputs: Pipeline stall and flush controls
// -----------------------
output logic if_stall_en,
output logic if_id_stall_en,
output logic id_ex_stall_en,
output logic ex_mem_stall_en,
output logic if_id_flush_en,
output logic id_ex_flush_en
);
// -----------------------
// Data Forwarding (EX-stage operand bypass)
// -----------------------
// Forwarding resolves RAW hazards by routing results from
// MEM or WB back to the EX ALU inputs. We compare EX-stage
// source register addresses (the instruction currently in EX)
// against the destination registers of instructions in MEM/WB.
//
// Priority: MEM > WB (MEM result is more recent).
// Operand A (ex_rs1_addr)
always_comb begin
if (mem_reg_write_en && (mem_rd_addr != 5'b0) && (mem_rd_addr == ex_rs1_addr)) begin
forward_a_sel = 2'b01; // Forward from MEM stage
end else if (wb_reg_write_en && (wb_rd_addr != 5'b0) && (wb_rd_addr == ex_rs1_addr)) begin
forward_a_sel = 2'b10; // Forward from WB stage
end else begin
forward_a_sel = 2'b00; // No forwarding
end
end
// Operand B (ex_rs2_addr)
always_comb begin
if (mem_reg_write_en && (mem_rd_addr != 5'b0) && (mem_rd_addr == ex_rs2_addr)) begin
forward_b_sel = 2'b01; // Forward from MEM stage
end else if (wb_reg_write_en && (wb_rd_addr != 5'b0) && (wb_rd_addr == ex_rs2_addr)) begin
forward_b_sel = 2'b10; // Forward from WB stage
end else begin
forward_b_sel = 2'b00; // No forwarding
end
end
// -----------------------
// Load-Use Hazard Detection
// -----------------------
// If the instruction in EX is a load, and the instruction in
// ID reads from the load's destination, we must stall for 1
// cycle because the load data is not available until MEM.
logic load_use_stall;
assign load_use_stall = ex_is_load && (ex_rd_addr != 5'b0) &&
((ex_rd_addr == id_rs1_addr) || (ex_rd_addr == id_rs2_addr));
// -----------------------
// Stall and Flush Signal Generation
// -----------------------
always_comb begin
// Defaults: no stalls, no flushes
if_stall_en = 1'b0;
if_id_stall_en = 1'b0;
id_ex_stall_en = 1'b0;
ex_mem_stall_en = 1'b0;
if_id_flush_en = 1'b0;
id_ex_flush_en = 1'b0;
// (1) Memory stall (D-Cache miss): freeze the entire pipeline
if (mem_stall_req) begin
if_stall_en = 1'b1;
if_id_stall_en = 1'b1;
id_ex_stall_en = 1'b1;
ex_mem_stall_en = 1'b1;
end
// (2) Load-Use stall: freeze IF and IF/ID, insert bubble into ID/EX
if (load_use_stall && !mem_stall_req) begin
if_stall_en = 1'b1;
if_id_stall_en = 1'b1;
id_ex_flush_en = 1'b1;
end
// (3) Branch/jump misprediction: flush IF/ID and ID/EX
// Branch flush overrides load-use stall (the stalled instruction is invalid anyway)
if (ex_branch_taken) begin
if_id_flush_en = 1'b1;
id_ex_flush_en = 1'b1;
// Cancel load-use stall if branch is also taken
if_stall_en = 1'b0;
if_id_stall_en = 1'b0;
end
end
endmodule
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